Authorities said Kathleen Ensz was so offended by repeated mailings from Rep. Marilyn Musgrave, Colorado Republican, that she stuffed one of those brochures into the mail slot of the congresswoman’s Greeley office wrapped around a little something extra: a dollop of doggy doo-doo.

Now Ms. Ensz, a 63-year-old retired University of Northern Colorado professor living in Greeley, is facing charges of misdemeanor “use of a noxious substance.” Her attorneys, however, argue that their client was exercising her constitutional right to free speech.

“Ms. Ensz’s action in returning an unwanted mailing from Congresswoman Musgrave … with a piece of feces was expressive conduct protected by the First Amendment,” Denver lawyer Patricia Bangert said in a motion filed Tuesday. The filing does not deny that Ms. Ensz put the package through the mail slot, claiming instead that the act is constitutionally protected and thus not a crime.

Critics denounced that argument as a load of bunk, but others say it’s not as far-fetched as it might seem. After all, if flag burning is protected under the Constitution, why not poop parceling?

“Sometimes there’s a fine line between conduct, which is not protected, and speech, which is,” Denver lawyer Scott Robinson said. “She could have said, ‘Marilyn Musgrave’s policies are excrement,’ and that would have been entirely protected. But if you include in that conduct canine waste, it moves it into the area of criminal conduct.”

Ms. Ensz also is raising a stink over Weld County District Attorney Ken Buck’s role in the matter. At Tuesday’s hearing, her attorney asked to remove Mr. Buck from the case, citing his involvement in local Republican politics.

Mr. Buck endorsed the congresswoman in her re-election bid last year, and defense attorneys said that compromised his objectivity in the case. The district attorney’s office had no comment, but Mrs. Musgrave’s office denied the suggestion.

“Ken Buck is a capable, honest, straightforward DA, and I’m sure he’s fully capable of prosecuting those he believes have broken the law,” said Guy Short, Mrs. Musgrave’s chief of staff.

An active Democrat, Ms. Ensz registered her protest May 31 after receiving one mailer too many from Mrs. Musgrave’s office. The owner of a German shepherd, she fetched what police called a “very nice piece of feces” from her back yard and wrapped the brochure around it, according to court documents.

She scratched out her street address on the mailer, but police were able to narrow it down to her block using the postal code and four-digit extension. After that, it was just a matter of determining which house had a dog. It didn’t hurt that Ms. Ensz had several political signs in her front yard in support of Mrs. Musgrave’s opponent, Democrat Angie Paccione.

The jury trial is scheduled for May 15. If convicted, Ms. Ensz could be sentenced to six months in jail and a fine ranging from $50 to $750.

Mr. Robinson predicted the trial would be a “close call,” but that the jury ultimately would find Ms. Ensz’s conduct more compelling than her efforts at freedom of expression. On the other hand, if the jury rules in her favor, it could lend a whole new meaning to the phrase “dirty campaign.”

“If she wins,” said Mr. Robinson, “it’ll be Halloween 365 days a year.”